This invention relates to instrusion sensing devices and in particular to a device which utilizes a mercury switch for activating an alarm upon receipt of an intrusion action. A mercury tilt switch is well known in the art and there is an abundance of known configurations. Typically, the switch consists of an envelope containing a pool of mercury and a pair of spaced apart electrodes. The pool of mercury is free to move within the envelope and can be directed into contact with the electrodes to provide a closed state or moved away from the electrodes to provide an opened state. A great many of such switches have at least two modes of operation, as described. When an electrical circuit is connected to the switch, the circuit can be controlled according to switch operation, as is known in the art.
Each mercury switch has a characteristic feature known as the "tilt angle", which describes the angle of movement or tilt of the switch that is necessary before the mercury will change position in the envelope and open or close the circuit. A mercury switch can be designed with different tilt angles according to the preference and needs of the user, although the more sensitive switches, i.e. those having small tilt angles, tend to be more expensive to construct.
The use of mercury tilt switches to detect movement of an object to which the switch is affixed is well known in the art. One such prior art device is disclosed in U.S. Pat. No. 3,818,161 entitled AUTOMATIC FLOATING GRAVITY CONTROLLED MERCURY SAFETY SWITCH WITH RESET MEANS issued on June 18, 1974 to J. Rickey, and involves the use of a mercury switch pivotally mounted so as to coact with a weighted ball that is responsive to the tilt of a vehicle chassis. A similar device, shown in U.S. Pat. No. 3,371,171 entitled IGNITION CUT-OFF DEVICE issued on Feb. 27, 1968 to C. Gregory, utilizes multiple chambers for the movement of mercury to produce the desired on-off effect when a vehicle chassis is tilted. U.S. Pat. No. 3,927,287 entitled SENSOR UNIT, issued on Dec. 16, 1975 to C. Hopwood, discloses a mercury switch encapsulated in a floating container which detects changes in the velocity of an automobile to which it is affixed.
Mercury switches have been employed in security systems to detect opening and closing of doors, windows and in various other configurations to detect movement, vibration as well as many other conditions. All such applications require associated structure cooperating with the switch to provide controlled operation according to the nature of the intrusion.
There are other types of vibration detecting devices that do not rely on mercury tilt switches. U.S. Pat. No. 4,185,180 entitled VIBRATION SENSING DEVICE, issued on Jan. 22, 1980 to F. Anderson discloses a pair of electrically conductive plates, each having an annular track and an electrically conductive bar mounted between the tracks and which leaves the surface of the tracks in response to external vibration.
In spite of the plethora of prior art devices, there is a need for an intrusion sensing device of a simple construction which can be activated by a plurality of different types of forces including vibration and translational and rotational movement and which can be adjusted to react to various magnitudes and directions of force without modification to the mercury switch itself.
Furthermore, mercury tilt switches are subject to being deactivated by the reverse of the movement that they are intended to detect, thus cutting off the signal or alarm that has been given. In many applications, it is furthermore desired to the user of an intrusion alarm device that the device, once "tripped", remains so in order that the user may determine the point of intrusion. To overcome this inherent deficiency in mercury tilt switches, prior art inventions have incorporated springs or other means such as electrical interlock devices to counteract the effect of the reversal of movement. Examples are contained in Rickey and Gregory described above. When such features are included, the intrusion switch necessarily becomes more costly and complicated to manufacture. In addition, counter-measures of the type described above are added sources of malfunctioning and require servicing in order to insure that the operational integrity of the system is maintained.
In order to overcome certain deficiencies of the prior art devices, it is the object of the present invention to provide an improved intrusion sensing device which is activated by various types of movement and once activated, will fall to a tripped position and will not become deactivated by a further or reverse movement.
It is a further object of this invention to provide an improved intrusion sensing device which is simple and economical to construct and requires minimal servicing.
Another object is to provide an intrusion sensing device that is easy to install and can be readily adjusted to detect forces of varying intensities.
A further object is to provide a sensing device employing a mercury switch wherein the tolerances of the switch as to tilt angle are circumvented, as the housing means associated with the switch assure desired operation.